1. Field of the Invention
The present invention relates to an image reading apparatus for reading an image of an original mounted on a platen glass and an image forming apparatus, such as a copying machine, for forming an image of an original image using the image reading apparatus equipped.
2. Description of the Prior Arts
In an image reading apparatus for reading an image of the original mounted on a platen glass using a scan optical system adapted to move, if image data thus read are used to expose the image as they are onto a photosensitive body, it is difficult to obtain an image having an appropriate density. In order to correct a density of a read original image, preliminary original reading which is referred to as pre-scan is performed prior to an actual scan operation, and image information obtained by reading part of the original image is used to perform density correction on the image data which are read by actual scanning.
Japanese Patent Application KOKAI Publication No. 9-289590 (1997-289590) describes an approach by which, in an analog copying machine, in reading of an original, a density of the original is read when a scan optical system moves to a reading operation start position. It also describes an approach of providing means for altering a scanner stop position in a standby state, to reserve an original scan region for reading of the original density.
In a digital copying machine, an image of an original is read by a CCD sensor, so that data of the read image are converted by an A/D conversion circuit into a digital signal. In the digital copying machine, the read image data need to undergo correction of a relationship between a quantity of light reflected by an exposure lamp and a white background level of the original, wherein specifically shading correction is performed by reading the white background level, which is a quantity of light reflected from a white background portion illuminated by the exposure lamp, which quantity provides a reference before original reading.
In the digital copying machine, it is necessary to perform a shading operation, a preliminary original reading operation for automatic original density determination (EE), or an APS operation for automatic original size determination, before the original image is read. Conventionally, shading correction has been performed for correcting the white background level, prior to the preliminary original reading operation for original density decision, which provides the reference of the original density. Therefore, it takes a long time before a primary original scan operation starts, resulting in a drawback that there causes poor operability in copying to a user.